Introduction to Mass Spectrometry (MS)

1. Introduction to Mass Spectrometry (MS) • A mass spectrometer produces a spectrum of masses based on the structure of a molecule. • The x-axis of a mass spectrum represents the masses of ions produced (m/z) • The y-axis represents the relative abundance of each ion produced • The pattern of ions obtained and their abundance is characteristic of the structure of a particular molecule 1

3. Ionization (the formation of ions) • A molecule is bombarded with a beam of high energy electrons • An electron is dislodged from the molecule by the impact, leaving a positively charged ion with an unpaired electron (a radical cation) • This initial ion is called the molecular ion (M+.) because it has the same molecular weight as the analyte 2

4. Fragmentation • Excess vibrational energy is imparted to the molecular ion by collision with the electron beam - this causes fragmentation • The fragmentation pattern is highly characteristic of the structure of the molecule 3

5. Fragmentation by Cleavage at a Single Bond • Cleavage of a radical cation gives a radical and a cation but only the cation is observable by MS • In general the fragmentation proceeds to give mainly the most stable carbocation • In the spectrum of propane the peak at 29 is the base peak (most abundant) 100% and the peak at 15 is 5.6% 11

6. Fragmentation Equations • The M+. Ion is formed by loss of one of its most loosely held electrons • If nonbonding electron pairs or pi electrons are present, an electron from one of these locations is usually lost by electron impact to form M+. • In molecules with only C-C and C-H bonds, the location of the lone electron cannot be predicted and the formula is written to reflect this using brackets 12

7. Example: The spectrum of hexane 13

8. Example: spectrum of neopentane • Fragmentation of neopentane shows the propensity of cleavage to occur at a branch point leading to a relatively stable carbocation • The formation of the 3o carbocation is so favored that almost no molecular ion is detected 14

9. Carbocations stabilized by resonance are also formed preferentially • Alkenes fragment to give resonance-stabilized allylic carbocations 15

10. Carbocations stabilized by resonance are also formed preferentially • Alkenes fragment to give resonance-stabilized allylic carbocations • Carbon-carbon bonds next to an atom with an unshared electron pair break readily to yield a resonance stabilized carbocation • Z=N, O, or S R may be H 15

11. Carbocations stabilized by resonance are also formed preferentially • Alkenes fragment to give resonance-stabilized allylic carbocations • Carbon-carbon bonds next to an atom with an unshared electron pair break readily to yield a resonance stabilized carbocation • Z=N, O, or S R may be H 15

12. Carbon-carbon bonds next to carbonyl groups fragment readily to yield resonance stabilized acylium ions 16

13. Carbon-carbon bonds next to carbonyl groups fragment readily to yield resonance stabilized acylium ions 16

14. Alkyl substituted benzenes often lose a hydrogen or alkyl group to yield the relatively stable tropylium ion • Other substituted benzenes usually lose their substitutents to yield a phenyl cation 17

15. Fragmentation by Cleavage of 2 Bonds • The products are a new radical cation and a neutral molecule • Alcohols usually show an M+.-18 peak from loss of water 18

16. The masses of molecular and fragment ions also reflect the electron count, depending on the number of nitrogen atoms in the species. Ions with no nitrogen or an even # N atoms Molecular Ion Fragment Ions odd-electron ions even-electron ions even-number mass odd-number mass Ions having an odd # N atoms Molecular Ion Fragment Ions odd-electron ions even-electron ions odd-number mass even-number mass

17. The masses of molecular and fragment ions also reflect the electron count, depending on the number of nitrogen atoms in the species. Ions with no nitrogen or an even # N atoms Molecular Ion Fragment Ions odd-electron ions even-electron ions even-number mass odd-number mass Ions having an odd # N atoms Molecular Ion Fragment Ions odd-electron ions even-electron ions odd-number mass even-number mass

19. http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/MassSpec/masspec1.htmhttp://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/MassSpec/masspec1.htm